Java中的正则表达式注释匹配代码无法正常工作

Question

I have this code for Identifying the comments and print them in java

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Solution {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile("(\\/\\*((.|\n)*)\\*\\/)|\\/\\/.*");
        String code = "";
        Scanner scan = new Scanner(System.in);
        while(scan.hasNext())
        {
            code+=(scan.nextLine()+"\n");

        }
        Matcher matcher = pattern.matcher(code);
        int nxtBrk=code.indexOf("\n");
        while(matcher.find())
        {

            int i=matcher.start(),j=matcher.end();
            if(nxtBrk<i)
            {
                System.out.print("\n");
            }
            System.out.print(code.substring(i,j));
            nxtBrk = code.indexOf("\n",j);

        }



    scan.close();
    }

}

Now when I try the code against this input

 /*This is a program to calculate area of a circle after getting the radius as input from the user*/  
\#include<stdio.h>  
int main()  
{ //something

It outputs right and only the comments. But when I give the input

 /*This is a program to calculate area of a circle after getting the radius as input from the user*/  
\#include<stdio.h>  
int main()  
{//ok
}  
/*A test run for the program was carried out and following output was observed  
If 50 is the radius of the circle whose area is to be calculated
The area of the circle is 7857.1429*/  

The program outputs the whole code instead of just the comments. I don't know what wrong is doing the addition of that last lines.

EDIT: parser is not an option because I am solving problems and I have to use programming language . link https://www.hackerrank.com/challenges/ide-identifying-comments

Answer 1

Parsing source code with regular expressions is very unreliable. I'd suggest you use a specialized parser. Creating one is pretty simple using antlr . And, since you seem to be parsing C source files, you can use the C grammar .

Answer 2

Your pattern, shorn of its Java quoting (and some unnecessary backslashes), is this:

(/\*((.|
)*)\*/)|//.*

That's fine enough, except that it has just greedy quantifiers which means that it will match from the first /* to the last */ . You want non-greedy quantifiers instead, to get this pattern:

(/\*((.|
)*?)\*/)|//.*

Small change, big consequence since it now matches to the first */ after the /* . Re-encoded as Java code.

Pattern pattern = Pattern.compile("(/\\*((.|\n)*?)\\*/)|//.*");

(Be aware that you are very close to the limit of what it is sensible to match with regular expressions. Indeed, it's actually incorrect since you might have strings with /* or // in. But you'll probably get away with it…)